Bioethanol was already commercialized in Brazil and the USA, and the production thereof is greatly increasing. However, it is produced from food biomass such as sugar canes or corn, and causes the instability of crop prices. Because an increase in the price of crops leads directly to an increase in the price of bioethanol, it can increase the fuel price and even can cause problems associated with fuel supply. For this reason, studies and investments on a variety of non-food biomass alternatives have been continued, and among them, woody biomass is a representative alternative.
Woody biomass (trees, grasses, agricultural residues such as rice straw and chaff, etc.) together with sugar biomass (sugar canes, sugar beets, etc.) and starch biomass (crops, potatoes, etc.) are used as biofuels that can produce energy such as methane, ethanol and hydrogen by pyrolysis and fermentation processes.
Woody biomass is generally composed of cellulose (40-50%), hemicellulose (25-35%) and lignin (15-20%), even though the compositions and contents of the chemical components of wood vary depending on the kind of tree (a needle-leaf tree or a broad-leaved tree), the age of trees, etc. Cellulose is a polymer compound consisting of glucose units regularly linked by hydrogen bonds and van der Waals forces, and hemicellulose is composed of pentoses, such as xylose and arabinose, linked in the β-1,4 configuration, and serves as an interface between cellulose and lignin. Also, lignin is an insoluble, non-degradable polymer compound with aromatic substances having phenylpropanoid units linked together irregularly, and is characterized by a structure that blocks the degradation of polysaccharides.
A technology of producing biofuels from woody biomass or perennial grassy biomass entails a problem in that the production cost of biofuels is increased due to a relatively high lignin content compared to that of starch (crop) biomass and sugar (sugar cane liquor) biomass.
Biofuel production processes of preparing bioethanol from cellulose are grossly divided into feedstock acquisition, pretreatment, saccharification, fermentation and purification processes. Particularly, the pretreatment process is performed when non-degradable woody biomass composed of cellulose, hemicellulose and lignin, linked in a complex and hard form, is used. The pretreatment process aims to effectively separate cellulose and hemicellulose from lignin having a complex and hard structure, and the separated cellulose is hydrolyzed into glucose, a representative monosaccharide, by microorganisms or enzyme in the saccharification process. When glucose is fermented, ethanol is produced.
As the pretreatment methods, many physical and chemical methods have been studied, including steam explosion, alkaline treatment, sulfur dioxide treatment, hydrogen peroxide treatment, supercritical ammonia treatment, ammonia freeze explosion, ammonia recycled percolation, and pyrochemical treatment methods.
The physical methods typically include milling and steam explosion. Milling is a method that grinds lignocellulosic particles into very fine particles by a milling device to induce the structural change of the lignocellulosic particles, and has disadvantages in that it consumes a large amount of energy and has low yield or saccharification rate. The steam explosion method is a method in which lignocellulose is steamed in an autoclave containing high-temperature steam for a certain time, and then the valve of the autoclave is instantaneously opened so that the structure of the lignocellulose is instantaneously opened, like popcorn, thereby providing a substrate with which an enzyme can easily come into contact.
The chemical methods typically include a method of treating lignocellulose with 2% (w/w) or less sulfuric acid, and a dilute-acid hydrolysis method in which lignocellulose is steamed with high-temperature steam at a temperature of 160˜200° C. for 1-10 minutes, like the steam explosion method, so that hemicellulose is hydrolyzed into monosaccharides and oligosaccharides by an acid catalytic reaction.
The physical pretreatment method has disadvantages in that the process proceeds slowly, a large amount of energy is consumed, and saccharification efficiency and economic efficiency are low. Also, the chemical pretreatment method has disadvantages in that, because a strongly acidic or strongly alkaline compound is used, a portion of pentose produced is decomposed into furfural acting as a fermentation inhibitor, and in that it is costly, is unsuitable for treatment of a large amount of lignocellulose, is toxic, corrodes equipment, and causes environmental pollution problems by waste.
U.S. Pat. No. 4,129,263 discloses an apparatus configured to refine biomass by feeding biomass between two rotating discs. Japanese Patent No. 3417802 discloses an apparatus configured to refine feedstock by feeding the feedstock between two rotating discs.
The discussion in the foregoing background section is to provide general background information, and does not constitute an admission of prior art.